1. Field of the Invention
The present invention relates to Digital Audio Broadcasting (DAB). More particularly, the present invention relates to a null detector of a DAB receiver and a corresponding method for null detection.
2. Description of the Related Art
DAB was developed as a research project for the European Union, which started in 1987 on initiative by a consortium formed in 1986. Traditionally radio programmes were broadcast on different frequencies via FM and AM, and the radio had to be tuned into each frequency. This used up a comparatively large amount of spectrum for a relatively small number of stations, limiting listening choice. On the other hand, DAB is a digital radio broadcasting system that through the application of multiplexing and compression combines multiple audio streams onto a single broadcast frequency. Consequently DAB has some benefits over and above traditional analog systems. For example, DAB radios automatically tune to all the available stations and offering a list of all stations. DAB is more bandwidth efficient than analogue for national radio stations. In addition, the DAB standard integrates features to reduce the negative consequences of multipath fading and signal noise, which afflict existing analogue systems. DAB has been established in some countries such as the United Kingdom, Denmark, and Norway.
In a DAB system, synchronization is essential for a receiver to decode the received signal correctly. Here synchronization means locating the beginning of each transmission frame in the received signal. Theoretically the first orthogonal frequency-division multiplexing (OFDM) symbol of a transmission frame is the null symbol and during the interval of the null symbol the received signal shall be equal to zero. In an ideal environment the null symbol may be used for synchronization. However, in the real world there are always signal fading, attenuation, noises and interferences, causing sudden drops in the received signal which are likely to be considered as null symbols by naive receivers. Therefore it is impractical to treat every quiet interval as the null symbol. In other words, it is desirable to have a practical and effective solution for synchronization in a DAB system.